1. Field of the Invention
The present invention relates to a superconducting magnet with a refrigerator and a magnetic resonance imaging apparatus using the same.
2. Description of the Related Art
An MRI (magnetic resonance imaging) apparatus shots images in a physical property and a chemical property of a subject by use of the nuclear magnetic resonance phenomenon experienced when a high frequency pulses is applied to a subject placed in a static magnetic field having homogeneous magnetic density distribution.
Such an MRI apparatus is strongly requested to have an increased performance and miniaturization from view point of wider, higher applications to medical treatments and installations at more medical facilities.
The MRI apparatus capable of generating a strong static magnetic field, which is one of such requirements, is known which includes a superconducting coil for circulate a persistent current and a refrigerant (liquid helium) to maintain a temperature of the superconducting coil lower than a critical temperature. There are many MRIs including a refrigerator for re-liquefying a vaporized refrigerant to suppress a consumption of the refrigerant. As the refrigerator, a regenerative refrigerator (for example, Gifford-McMahon (GM) refrigerator) is known.
The regenerative refrigerator performs a cooling function by use of adiabatic expansion generated by reciprocation motions of a regenerative refrigerant having a large specific heat at ultracold temperatures (for example, HoCu2). Generally, such a regenerative refrigerant is magnetized itself, which distorts the static magnetic field generated in the MRI apparatus as well as the reciprocating motions may cause a periodic variation in evenness of the static magnetic field in an imaging area.
To solve these problems the following technologies are disclosed:
JP 10-165388A discloses a technology in which a canceling coil for canceling distortion in a magnetic field caused by a magnetized regenerative refrigerant is installed and the canceling coil is driven in accordance with displacement to the regenerative refrigerant.
Further, JP 9-326513A discloses a technology in which thin superconducting sheets are laminated and arranged around the regenerative refrigerant to shield the static magnetic field.
Further, JP 2001-263844A discloses a structure in which the regenerative refrigerant is surrounded with an electric good conductor which shields a dynamic magnetic field generated by the reciprocating motion of the regenerative refrigerant.
Further, JP 10-213357A discloses a structure in which a refrigerator is surrounded with a superconducting material of BiPb system (bismuth lead) which shields the dynamic magnetic field.
Further, JP 2000-266417A discloses a structure in which a superconducting coil is wound around an outer circumference of the regenerative refrigerant of the refrigerator to form a closed loop to shield the dynamic magnetic field.
Further, JP 3122539 discloses a structure in which a superconducting material is arranged at a structural member on a side of the superconducting magnet where the refrigerator is mounted to shield the dynamic magnetic field.